These instructions are extracted from part of a Microsoft Technical note - the original is available 
at http://support.microsoft.com/support/kb/articles/Q150/6/29.asp

To configure PERL script mapping

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk. 

Start Regedt32.exe and open the following registry key: 

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W3SVC \Parameters\ScriptMap 

Click Add Value from the Edit menu. 

The Value Name is .pl ''(.cgi for Wiki builders)''

The Data type is REG_SZ. 

The String value is <the full path to perl.exe>\perl.exe %s %s 

NOTE: The "%s %s" is case sensitive. (e.g. "%S %S" will not work). 

''Restart the WWW service. ''